1. Field of the Invention
The present invention relates to display devices having a display screen on which characters and images are displayed, and relates to a technique for improving visibility of a display screen.
2. Description of the Related Art
In recent years, a so-called self-luminous display device has been attracting attention, which has pixels formed with a light-emitting element such as a light-emitting diode (LED). As a light-emitting element used in such a self-luminous display device, there is an organic light-emitting diode (also called an OLED, an organic EL element, an electroluminescence (EL) element, or the like), which has been attracting attention and used for an EL display (e.g., an organic EL display). Since the light-emitting element such as an OLED is a self-luminous type, it is advantageous, as compared to a liquid crystal display, in that high visibility of pixels is ensured, no backlight is required, high response speed is achieved, and the like. The luminance of a light-emitting element is controlled by the value of current flowing therein.
As a driving method of controlling gray scales of light emission (luminance) in such a display device, there are a digital gray scale method and an analog gray scale method. In the digital gray scale method, gray scales are expressed by controlling ON/OFF of a light-emitting element in a digital manner. On the other hand, as for the analog gray scale method, there are a method of controlling the light-emission intensity of a light-emitting element in an analog manner, and a method of controlling the light-emission time of a light-emitting element in an analog manner.
In the digital gray scale method, only two states of a light-emitting element can be selected, which are a light-emission state and a non-light-emission state; therefore, only two gray scales can be expressed. Thus, the digital gray scale method is often used in combination with another method to achieve multi-grayscale display. As a method for achieving multi-grayscales, a time gray scale method is often used in combination (Reference 1: Japanese Patent Laid-Open No. 2001-324958 and Reference 2: Japanese Patent Laid-Open No. 2001-343933). In addition, an area gray scale method is used in some cases (Reference 3: Japanese Patent Laid-Open No. 2001-125526).
On the other hand, a pixel configuration for displaying clear images and characters both has been developed (Reference 4: Japanese Patent Laid-Open No. 2005-062416). In general, a delta array or a stripe array is adopted as a pixel configuration; however, in Reference 4, a hexagonal pixel configuration is adopted.